Building uImage

    u-boot是不支持zImage等linux内核格式的，所以你要用u-boot来引导linux的话，你就要建立一个u-boot能识别的linux内核格式来。u-boot有自己可引导的内核的格式－－－uImage，我不知道它确确的叫法，没有具体去查过，我这里先就叫它uImage吧。在linux 2.6以上的内核中，你可以用make uImage这样的语句去直接生成它，但是你是用linux 2.4或者更早的linux的话，你就要自己手动去生成它了。

生成uImage格式的办法很简单，只有几步而已：
1.生成linux.bin,纯二进制的linux内核，用以下命令：（这里假定你已经知道如何生成内核）
arm-linux-objcopy –O binary -R .note –R .comment -S vmlinux linux.bin

2.压缩内核
gzip -9 linux.bin

3.生成uImage格式内核（假定你会并且已经u-boot，mkimage就在$uboot/tools中）
mkimage –A arm –O linux –T kernel –C gzip –a 0x30008000 –e 0x30008000 –n “Linux Kernel Image” –d linux.bin.gz uImage

以上只是一个简单的过程，照着做的话应该是能够成功的。你还可以参照u-boot根目录下的README文件，在它里面有相应的描述。下面这段话来自u-boot官方的README：

Building a Linux Image:
-----------------------

With U-Boot, "normal" build targets like "zImage" or "bzImage" are
not used. If you use recent kernel source, a new build target
"uImage" will exist which automatically builds an image usable by
U-Boot. Most older kernels also have support for a "pImage" target,
which was introduced for our predecessor project PPCBoot and uses a
100% compatible format.

Example:

    make TQM850L_config
    make oldconfig
    make dep
    make uImage

The "uImage" build target uses a special tool (in 'tools/mkimage') to
encapsulate a compressed Linux kernel image with header     information,
CRC32 checksum etc. for use with U-Boot. This is what we are doing:

* build a standard "vmlinux" kernel image (in ELF binary format):

* convert the kernel into a raw binary image:

    ${CROSS_COMPILE}-objcopy -O binary \
                 -R .note -R .comment \
                 -S vmlinux linux.bin

* compress the binary image:

    gzip -9 linux.bin

* package compressed binary image for U-Boot:

    mkimage -A ppc -O linux -T kernel -C gzip \
        -a 0 -e 0 -n "Linux Kernel Image" \
        -d linux.bin.gz uImage


The "mkimage" tool can also be used to create ramdisk images for use
with U-Boot, either separated from the Linux kernel image, or
combined into one file. "mkimage" encapsulates the images with a 64
byte header containing information about target architecture,
operating system, image type, compression method, entry points, time
stamp, CRC32 checksums, etc.

"mkimage" can be called in two ways: to verify existing images and
print the header information, or to build new images.

In the first form (with "-l" option) mkimage lists the information
contained in the header of an existing U-Boot image; this includes
checksum verification:

    tools/mkimage -l image
      -l ==> list image header information

The second form (with "-d" option) is used to build a U-Boot image
from a "data file" which is used as image payload:

    tools/mkimage -A arch -O os -T type -C comp -a addr -e ep \
              -n name -d data_file image
      -A ==> set architecture to 'arch'
      -O ==> set operating system to 'os'
      -T ==> set image type to 'type'
      -C ==> set compression type 'comp'
      -a ==> set load address to 'addr' (hex)
      -e ==> set entry point to 'ep' (hex)
      -n ==> set image name to 'name'
      -d ==> use image data from 'datafile'

Right now, all Linux kernels for PowerPC systems use the same load
address (0x00000000), but the entry point address depends on the
kernel version:

- 2.2.x kernels have the entry point at 0x0000000C,
- 2.3.x and later kernels have the entry point at 0x00000000.

So a typical call to build a U-Boot image would read:

    -> tools/mkimage -n '2.4.4 kernel for TQM850L' \
    > -A ppc -O linux -T kernel -C gzip -a 0 -e 0 \
    > -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/ppc/coffboot/vmlinux.gz \
    > examples/uImage.TQM850L
    Image Name:   2.4.4 kernel for TQM850L
    Created:      Wed Jul 19 02:34:59 2000
    Image Type:   PowerPC Linux Kernel Image (gzip compressed)
    Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
    Load Address: 0x00000000
    Entry Point:  0x00000000

To verify the contents of the image (or check for corruption):

    -> tools/mkimage -l examples/uImage.TQM850L
    Image Name:   2.4.4 kernel for TQM850L
    Created:      Wed Jul 19 02:34:59 2000
    Image Type:   PowerPC Linux Kernel Image (gzip compressed)
    Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
    Load Address: 0x00000000
    Entry Point:  0x00000000

NOTE: for embedded systems where boot time is critical you can trade
speed for memory and install an UNCOMPRESSED image instead: this
needs more space in Flash, but boots much faster since it does not
need to be uncompressed:

    -> gunzip /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/ppc/coffboot/vmlinux.gz
    -> tools/mkimage -n '2.4.4 kernel for TQM850L' \
    > -A ppc -O linux -T kernel -C none -a 0 -e 0 \
    > -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/ppc/coffboot/vmlinux \
    > examples/uImage.TQM850L-uncompressed
    Image Name:   2.4.4 kernel for TQM850L
    Created:      Wed Jul 19 02:34:59 2000
    Image Type:   PowerPC Linux Kernel Image (uncompressed)
    Data Size:    792160 Bytes = 773.59 kB = 0.76 MB
    Load Address: 0x00000000
    Entry Point:  0x00000000


Similar you can build U-Boot images from a 'ramdisk.image.gz' file
when your kernel is intended to use an initial ramdisk:

    -> tools/mkimage -n 'Simple Ramdisk Image' \
    > -A ppc -O linux -T ramdisk -C gzip \
    > -d /LinuxPPC/images/SIMPLE-ramdisk.image.gz examples/simple-initrd
    Image Name:   Simple Ramdisk Image
    Created:      Wed Jan 12 14:01:50 2000
    Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
    Data Size:    566530 Bytes = 553.25 kB = 0.54 MB
    Load Address: 0x00000000
    Entry Point:  0x00000000
   
    看到这么长的E文，除非你的英文很好，不然的话就像我一样晕菜了！^-^